In radio transmitters, a transmission signal, i.e. the signal being transmitted, is amplified in a radio frequency power amplifier which amplifies the transmission signal to a level suitable for transmission over an air interface to a radio receiver. The level of the power-amplified transmission signal should be high enough to enable the radio receiver to decode information contained in the transmission signal.
Power amplifiers are not ideal components and thus power amplification does not result in an ideally power-amplified transmission signal. Instead, the power-amplified transmission signal is corrupted by amplitude and phase distortion caused by the power amplifier. If this distortion is not corrected before transmission or at the radio receiver, the decoding of the information will be hindered at the radio receiver.
The amplitude and phase distortion caused by the power amplifier may be compensated by predistorting the transmission signal before the power amplification. One known transmission signal predistortion method monitors constantly the amplitude and phase distortion affected by the power amplifier. Accordingly, the solution comprises a feedback loop for the power-amplified transmission signal to enable measurement of the amplitude and phase distortion caused by the power amplifier. On the basis of the measured distortion values, predistortion values are calculated for the amplitude and phase of the transmission signal, and the transmission signal is predistorted with these predistortion values before power amplification. This solution ensures that the predistortion values are always up-to-date, but the solution requires excessive amount of signal processing which requires high computational capacity and consumes power. Both of these issues are critical in a mobile communication device which operates with a battery and is desired to be compact in size. Therefore, there is a need for simpler, yet efficient, predistortion solutions.